1. Field of the Invention
The present invention relates to a mold structure used to produce a discrete track medium, an imprinting method, a method for producing a discrete track medium, and a discrete track medium.
2. Description of the Related Art
In recent years, more and more hard disk drives superior in terms of speed and cost have been installed as popular storage devices in portable machines such as cellular phones, compact acoustic devices and video cameras.
Along with the increase in the sharing of hard disk drives as storage devices installed in portable machines, there is a need to meet the demands for further compactness and increase in capacity, and techniques for improving recording density are being asked for.
To increase the recording density of the hard disk drives, methods for narrowing the spaces between data tracks in magnetic recording media or methods for narrowing the widths of magnetic heads have been conventionally employed.
However, the narrowing of the spaces between the data tracks causes effects of magnetism between adjacent tracks (crosstalk) and/or effects of demagnetization by heat fluctuation to be noticeable, so that the increase in recording density has been limited.
Also, the increase in surface recording density by narrowing the widths of the magnetic heads has been limited as well.
Accordingly, as a means to reduce noise caused by crosstalk, magnetic recording media called “discrete track media (DTM)” have been proposed. The discrete track media (DTM) reduce magnetic interference between adjacent tracks by having discrete structures in which nonmagnetic guard band areas are provided between the adjacent tracks so as to magnetically separate the tracks from each other.
As shown in FIG. 7, a conventional discrete track medium (DTM) includes a disc-shaped magnetic recording medium 204 having a circular hole 203 and a magnetic recording area 202 placed around the circular hole 203. The magnetic recording area 202 includes a servo pattern 201 placed so as to extend in a radial manner from an area near the circular hole 203 toward the periphery of the magnetic recording medium 204 and also includes a track pattern 200 provided in circles in a manner that is substantially concentric with the circular hole 203. This track pattern 200 has such a discrete structure as described above.
For production of the discrete track medium (DTM), an imprinting method (imprinting process) is used in which a desired pattern is transferred to an imprint resist layer formed over the surface of a magnetic recording medium, using a mold for forming a resist pattern (hereinafter also referred to as “mold” or “resist pattern forming mold”) (refer to Japanese Patent Application Laid-Open (JP-A) No. 2004-221465).
Specifically, this imprinting method is a method for obtaining a desired magnetic recording medium, which includes applying as an imprint resist a thermoplastic resin, a photocurable resin or a thermosetting resin onto a base material to be processed, closely attaching a mold, which has been processed to have a desired shape (pattern), to the applied resin under pressure, curing the resin by heating and subsequent cooling or by light irradiation, separating the mold from the resin so as to form on the resin a pattern which corresponds to the pattern formed on the mold, and subjecting the base material to patterning by dry or wet etching using this pattern as a mask.
A resist pattern forming mold used in such an imprinting method can be exemplified by a mold structure 300 including a plurality of line pattern forming concave portions 301 for forming the servo pattern 201 and the track pattern 200 (hereinafter, these patterns will also be referred to as “line patterns”), the line pattern forming concave portions 301 being provided on the side of an imprint surface corresponding to the magnetic recording area 202, as shown in FIG. 8A with an enlarged drawing for showing a part of the mold.
By pressing the mold structure 300 against an imprint resist layer and then separating (releasing) the mold structure, a pattern formation 320 composed of convex portions 310 as a line pattern which correspond to the line pattern forming concave portions 301 is obtained (see FIG. 8B).
As a method of separating the mold structure 300 from the imprint resist layer, there is a first separating method in which separation takes place from the peripheral side toward the center of the mold structure 300 as shown in FIG. 9A.
In FIG. 9A, first there is a state in which the mold structure 300 has been pressed and closely attached onto the imprint resist layer (shown by the color black in FIG. 9A), then separation takes place from the peripheral side toward the center of the mold structure 300 (this separation takes place in the order shown by the arrow direction in FIG. 9A). At this time, the direction of the separation is perpendicular to a line direction (pattern direction) of the track pattern 200.
Meanwhile, there is a second separating method in which separation takes place in one direction from the peripheral side of the mold structure 300 as shown in FIG. 9B.
In FIG. 9B, first there is a state in which the mold structure 300 has been pressed and closely attached onto the imprint resist layer (shown by the color black in FIG. 9B), then separation takes place from the periphery of the mold structure 300 on one side toward the periphery of the mold structure 300 on the opposite side (this separation takes place in the order shown by the arrow direction in FIG. 9B). At this time, there are places where the direction of the separation is perpendicular to the line direction of the track pattern 200, and also there are places where the direction of the separation is perpendicular to the line direction of the servo pattern 201.
Thus, when the mold structure 300 is separated (released) from the imprint resist layer, there are places where the direction of the separation of the mold structure 300 is perpendicular to the line direction of the convex portions 310 as a line pattern, and the convex portions 310 are pressurized from the direction of the separation of the mold structure 300, so that pattern defects such as collapse, detachment, missing, etc. of the convex portions easily arise, which is problematic.
In light of the foregoing, a pattern forming method has been disclosed as a means for reducing the occurrence of pattern defects, in which the separation (release) takes place, as the angle θ formed between the direction of separation and the line direction of a convex line pattern is set between −5° and 5° (refer to JP-A No. 2007-296683).
However, since the discrete track medium (DTM) has the servo pattern 201 extending in a radial manner and the concentrically formed track pattern 200 (see FIG. 7), there is such a problem that separation (release) with the angles θ between the direction of separation and the line directions of the patterns being all set between −5° and 5° is impossible (see FIGS. 9A and 9B).
Meanwhile, there has been disclosed a pattern forming method for reducing the occurrence of pattern defects by allowing the speed and angle at which a mold structure is released from an imprint resist layer to be variable (refer to JP-A No. 2008-183731).
However, even if the speed and the angle are optimized by making them variable, there is still such a problem that pattern defects such as collapse, detachment, missing, etc. of convex portions in pattern(s) easily arise depending upon the material for the resist layer, the thickness of the resist layer applied, the deign of pattern(s) (notably a convex line pattern with a high aspect ratio), etc.
Hence, satisfactory means for enabling imprinting on discrete track media (DTM) without causing pattern defects have not been obtained in reality.